In the field of optical fiber communication systems, a bidirectional communication system permitting the transmission of optical signals also from the subscriber (home) side is under study. In order to construct such an optical fiber communication system, a surface-mounted module connecting a light receiving/emitting device and an optical fiber is required, and conventionally a light receiving/emitting device and an optical fiber are coupled in the manner described below, for example.
In the case of coupling a laser diode (hereinafter referred to as "LD") to an optical fiber, a mark used for mounting the LD with high accuracy is formed on a silicon substrate, and a V-groove for positioning an optical fiber is also formed in the silicon substrate such that the position thereof relative to the mark is on the submicron order. The LD and the optical fiber are coupled to each other on the substrate.
In this case, a silicon substrate is used for the following reasons. First, a silicon substrate and an optical fiber have an identical coefficient of thermal expansion, and thus the two are scarcely displaced from each other due to temperature changes. Secondly, the mark used for mounting an LD and the V-groove for positioning an optical fiber can be formed on a silicon substrate with high accuracy by lithography, anisotropic etching or the like. Thirdly, a silicon substrate transmits near infrared radiation therethrough, so that the mounting positions of the LD and optical fiber can be observed from the underside of the substrate.
There is also conventionally known a method in which a lensed fiber having a convexly curved end face obtained by fusing an end portion thereof is coupled to an LD. In the case of using a lensed fiber, however, it is necessary that the space between the LD and the lensed fiber should be set to 5 to 10 .mu.m and also that the allowable positioning error of the LD in a transverse direction perpendicular to the optical axis of the lensed fiber should be 1 .mu.m or less.
Meanwhile, where a silicon substrate is treated with high precision by lithography, an expensive machine is required and also the substrate must be handled with care during the treatment. On the other hand, where anisotropic etching is used for the treatment, the orientation of the silicon substrate must be set with high accuracy relative to the positioning of an etching mask, and also the etching conditions must be controlled with precision.
Therefore, in either of the above two methods, if the treatment conditions are improperly set, then the aforementioned marks and V-grooves of treated silicon substrates are subject to positional variations, lowering the yield. As a consequence, a problem arises in that the products (surface-mounted modules) are costly.
Silicon substrates may alternatively be subjected to mechanical machining; in this case, however, the productivity is low and the machining cost is high, with the result that the produced surface-mounted modules are too expensive to be put to home use.
Meanwhile, an LD unavoidably generates heat since a high current is passed therethrough per unit area. The silicon substrate, however, is poor in heat conductivity as compared with metal and thus has low heat dissipation property. Accordingly, with a surface-mounted module using a silicon substrate, the light output of the LD becomes saturated at a low current value, giving rise to a problem that the light output cannot be increased.
Further, in a conventional surface-mounted module using a lensed fiber, if a material having a large coefficient of thermal expansion is used for the substrate, the LD and the lensed fiber can come into contact with each other due to temperature changes, possibly damaging these elements. Since the aforementioned allowable positioning error is 1 .mu.m or less, moreover, displacement in the transverse direction occurring when the LD and the lensed fiber are coupled results in variations in the coupling efficiency, causing a reduction in the yield of assembled surface-mounted modules as well as an increase of the manufacturing cost.